Abstract Background: Recent studies suggest that tumor shrinkage as measured by RECIST 1.1 criteria may not be adequate to assess early signs of anti-tumor activity in phase I trials. The value of Tumor Growth Rate (TGR) evaluation in this context has been suggested. Methods: We analyzed the TGR of solid tumors from patients prospectively enrolled in a dose-escalation phase I trial investigating the pan-CDK inhibitor BAY1000394 on a 3 days on/4 days off 21 day treatment cycle. Only patients who had tumor imaging evaluation at least one month before the onset of the experimental treatment, a baseline tumor imaging, and at least one tumor evaluation after 2 cycles of treatment were eligible. Patient consent was obtained prior to analysis. We computed the TGR across the pre- (REFERENCE) and post-treatment (EXPERIMENTAL) evaluation, as described by Ferté et al (Clin Cancer Res, 2013) and compared these results to RECIST-based response evaluation. Each patient was used as his/her own control.Results: A total of twelve patients were eligible for both RECIST and TGR evaluation. At the time of first evaluation, 9 patients were classified as progressive disease (PD) and 3 as stable disease (SD) using RECIST. Conversely, TGR decreased in 6 out of 12 patients (median: - 9.8 %; CI 95% - 27.4 to 2.3), suggesting antitumor activity by the investigational drug. However, these 6 patients were classified as PD due to the occurrence of new lesions and then discontinued from the trial. They had small cell lung, colorectal, thymic, and bladder cancer. Interestingly, two patients suffering from ovary adenocarcinoma with durable SD (10 treatment cycles) had almost no tumor growth rate change between the REFERENCE and the EXPERIMENTAL period, suggesting no anti-tumor activity, and could potentially have been discontinued earlier from the trial. Conclusion: Computing the variation of TGR in the context of a phase 1 trial investigating a pan-CDK inhibitor reveals early signs of antitumor activity in specific patients, not previously identified by RECIST. Its use to guide the “go/no go” decision-making in the early drug development context is promising. Citation Format: Antoine Hollebecque, Charles Ferte, Gerard Nitenberg, Mathieu Felices, Alexandre Durand-Salmon, Matthias Ocker, Jean-Charles Soria. Computing tumor growth rate across pre- and post-treatment periods uncovers anti-tumor activity in patients treated by a pan-CDK inhibitor (BAY1000394). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 900. doi:10.1158/1538-7445.AM2014-900